The liquid-crystal polyesters are rigid polymers unlike well-known thermoplastic polyesters such as polybutylene terephthalate or polyethylene terephthalate, and their molecular chains are difficultly bendable and keep their rodlike shapes even in a molten state. Therefore, the molecules are entangled with each other only to a small extent even in a molten state and oriented in one direction by even only a slight shearing force. They are thus crystalline even in the liquid form, namely, they are liquid crystals.
Although the liquid-crystal polyester can be molded by an ordinary injection molding process and has excellent moldability, heat resistance and dimensional stability, it has a defect that the surface layer of a molding prepared from it peels off to cause a fuzzing due to the strong orientation thereof. Thus it cannot be subjected to a surface processing such as vacuum deposition, ion plating or sputtering.
Although it might be thought possible to roughen its surface by pretreatment with a chemical used for treating ordinary resins, the surface of the molding thus prepared is chemically quite inert and has no affinity with any solvent. Thus it is impossible to roughen the surface by removing the oriented surface layer. Another method of roughening the surface comprises adding an inorganic filler for weakening the strong orientation of the liquid-crystal polyester resin or an additive which can be easily leached and treating the surface with a strong acid or alkali solution. However, when a conductive circuit is formed on the etched surface by, for example, wet plating according to electroless plating, a fragile oxide film is formed between the metallic coating and the material by water incorporated in the wet step and impurities contained in the solution used for the treatment, thereby reducing the adhesion of the metallic coating to the material. Thus, a higher surface roughness is necessary to obtain a higher adhesion. In this case, a resist ink for forming a circuit pattern is attached to only part of the surface because of the unevenness of the surface of the base plate entirely coated with the metal, and in the pattern-sticking method with a dry film, the adhesion of the film to the base plate is poor. Since the thickness of the metallic coating formed on a highly rough surface is uneven, the formation of a precision fine-line circuit by the subtractive method is difficult.
When a metallic coating is directly formed on a base plate improved in only orientation by vacuum deposition, sputtering or ion plating, the adhesion of the coating is poor, though a smooth surface can be obtained. Thus this process is impractical. Particularly when an ordinary thermoplastic resin is subjected to vacuum deposition, sputtering or ion plating, a gas is generated in too large an amount from the base material in vacuo to allow a metallic coating having excellent properties to adhere to the resin firmly. Although the liquid-crystal polyester is substantially free from the problem of the gas formation, no metallic coating can be firmly adhered thereto for the above-described reasons. Even when a precision fine-line circuit is to be formed by the subtractive method on a board prepared by adding an inorganic filler readily soluble in an acid or alkali to the resin, etching the resulting resin with the acid or alkali and subjecting the etched resin to an ordinary electroless plating to form a metallic coating on the entire surface, the minimum possible line width is 0.30 mm and the minimum possible space width is 0.30 mm in a circuit produced by the conventional process. In addition, when the surface roughness is improved in order to increase the fineness of the lines, the adhesion of the coating film thereto is reduced impracticably.
The terms "line width" and "space width" as used herein refer to the width of each line of the circuit and the space between the lines adjacent to each other, respectively.
The liquid-crystal polyester has features such as a low coefficient of linear expansion comparable to the coefficient of thermal linear expansion of an ordinary metal and a thermal resistance which causes no trouble even when it is dipped in a soldering bath at 260.degree. C. for 10 sec. No satisfactory process has been found yet, though investigations have been made for the purpose of developing a process for coating the surface of a molding with a metal in order to improve the above-described surface properties and to produce parts jointed to a metal with an adhesive or molding plated with a metal, taking advantage of these properties of the liquid-crystal polyester.